Hitherto, in a transfer portion of a CCD image sensor, for example, a poly-Si (polycrystalline silicon) film is used for constituting a transfer-accumulation electrode disposed on a buried CCD register. However, since the resistance of the poly-Si film is high, in the case where clock rate is high or where transmission distance is long, propagation of a signal transmitted through the transfer-accumulation electrode would be delayed. Particularly, it is difficult to obtain an image pickup device with a higher driving speed and a larger area. In order to solve such a problem, a shunt structure has been used in which a shunt wiring is disposed above the upper side of the transfer-accumulation electrode and a clock signal is supplied to the transfer-accumulation electrode through the shunt wiring, to thereby obviate the clock signal propagation delay and to achieve an image pickup device with a higher speed and a larger area. As the material of the shunt wiring, for example, a high melting point metal such as tungsten, molybdenum, etc., aluminum, or the like is used. In addition, in such an image sensor, a light shielding film for preventing the incidence of light on other regions than a photo-sensor is disposed. For example, a high melting point metal is used as the material of the light shielding film.
FIG. 7 is a partially sectional view showing the structure of a portion at and around a pixel of a CCD image sensor using the above-mentioned shunt structure, illustrating an example in which a buffer wring is provided between the transfer-accumulation electrode and the shunt wiring. In FIG. 7, a photo-sensor (not shown) and a CCD vertical transfer register (not shown) for constituting an image pickup pixel are provided in the inside of a Si substrate 10. Transfer-accumulation electrodes 14 and 16 in a two-layer structure are laminated above the upper surface of the Si substrate 10 in a region corresponding to the CCD vertical transfer register, with an insulating film 12 such as a silicon oxide film therebetween, and the buffer wiring 18 is provided above the upper surface thereof. The transfer-accumulation electrodes 14 and 16 are formed of poly-Si, and the buffer wiring 18 is formed of poly-Si or polycide (a two-layer structure film composed of polycrystalline silicon and a silicide [a silicon compound of a high melting point metal]). Incidentally, an insulating film between the transfer-accumulation electrodes 14, 16 and the buffer wiring 18 is provided with a contact window at an appropriate position, and the buffer wiring 18 is selectively connected to the transfer-accumulation electrodes 14, 16 through the contact window.
A shunt wiring 22 is provided above the upper side of the transfer-accumulation electrodes 14, 16 and the buffer wiring 18, with an insulating film 20 therebetween, and a light shielding film 26 is provided above the upper side of the shunt wiring 22, with an insulating film 24 therebetween. The shunt wiring 22 and the light shielding film 26 are formed of the above-mentioned high melting point metal or aluminum or the like. In addition, though omitted in FIG. 7, the insulating film 20 between the shunt wiring 22 and the buffer wiring 18 is provided with a contact window at an appropriate position, and the shunt wiring 22 is selectively connected to the buffer wiring 18 through a contact formed of a high melting point metal, which is provided in the contact window. Above the upper side of this assembly, an on-chip microlens 28 is laminated, with an upper insulating film, an in-layer lens functioning also as a planarizing film, and an on-chip filter therebetween. Incidentally, in a shunt structure that does not include the buffer wiring 18, the shunt wiring 22 and the transfer-accumulation electrodes 14, 16 are directly connected to each other by the contact window and the contact.
However, in the conventional shunt structure mentioned above, in the case where a high melting point metal is used as the material of the shunt wiring, there is the problem that a heat treatment conducted after the formation of the insulating film above the light shielding film 26 would raise the contact resistance between the shunt wiring 22 and the transfer-accumulation electrodes 14, 16 or the buffer wiring 18. On the other hand, where aluminum is used as the material of the shunt wiring, recovery of faults in the substrate by the heat treatment after the formation of the insulating film above the light shielding film 26 cannot be conducted, a dark current is increased, and it is necessary to enlarge the thickness of the inter-layer insulating film, so that the film thickness from the surface of the Si substrate 10 to the top end of the light shielding film 26 is increased, and the light utilization efficiency is lowered.
As a countermeasure against the above problems, a proposal has been made in which the shunt wiring 22 is composed of a laminate film of a high melting point metal nitride or oxide layer and a high melting point metal layer thereon. This constitution ensures that, even where a heat treatment for recovery of faults in the substrate is conducted, for example, after the formation of the insulating film above the light shielding film 26, the volume expansion due to the reaction between the high melting point metal layer and the silicon material disposed with the high melting point metal nitride or oxide layer therebetween is prevented, so that it is possible to prevent the rise in the contact resistance between the shunt wiring 22 and the transfer-accumulation electrodes 14, 16 or the buffer wiring 18 (see Japanese Patent Laid-open No. 2001-135811).
In the solid state image pickup device constituted as above, however, although the heat treatment-induced volume expansion between the shunt wiring formed by use of the high melting point metal and the transfer-accumulation electrodes or the buffer electrode formed by use of silicon can be prevented, the high melting point nitride or oxide layer having a high resistance is left between the high melting point metal and silicon. Therefore, there has been a limit to the suppression of the contact resistance between the high melting point metal and silicon.